<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div id="app">
        <ul>
            <li v-for="item of arr" v-if="flag">{{item}}</li>
        <!-- 输出四个空标签li -->
        </ul>
    </div>
</body>
<script src="./lib/vue.js"></script>
  <script>
    new Vue({
      data(){
        return {
          arr:['a','b','c','d'],
          flag:false
        }
      }
    }).$mount('#app')
  </script>
</html>